The detection of the three-dimensional spatial shape of bodies or body parts, in particular of human body parts, such as legs, torso or feet, is an important aspect in the production or assignment of fitting articles of clothing, orthopedic aids such as compression stockings, prostheses and ortheses and also in the production or assignment of fitting shoes. Numerous optical 3D scanners are on the market, most of which operate either on the basis of the methods of laser triangulation (see, e.g., PEDUS foot scanner of the company of Vitronic Dr. Stein, www.vitus.de) or stripe projection (see, e.g., bodyScan of the Breuckmann company, www.breuckmann.com). Both methods are based on triangulation, i.e. a stable spatial triangular arrangement of a light projector, a camera and a body for point-by-point determination of the distance of the body surface observed from the triangulation arrangement made up of the camera and the light projector, also referred to as measuring head. An XYZ point model of the surface viewed is determined from the sum of this distance data. In order to detect the entire body, either a plurality of camera/projector arrangements need to be mounted and/or moved around the body (e.g., in the case of the bodyScan of the Breuckmann company) or a camera/projector arrangement needs to be mechanically moved over the body surface (as in the case of the PEDUS foot scanner of the Vitronic Dr. Stein company, for example).
The angular arrangement of the camera/projector is sensitive: even small angle errors result in large errors of measurement in the distances measured. The movement of the triangulation arrangement in the space is equally sensitive: small errors in the position determination of the measuring head result in large errors of measurement in the 3D point model generated. This sensitivity results in that, even in case of a very sturdy and involved opto-mechanical construction, these scanners require frequent recalibration, in particular also after each transportation and upon each movement of the scanner. In addition, since these scanners frequently also carry the weight of the customer (e.g., in the case of the pedus foot scanner), the requirement of a rigid design can only be met with considerable expense, so that under this aspect as well, calibration needs to be repeated frequently.
Calibration of a 3D scanner operating on the basis of triangulation using laser or stripe projection with the aid of different normal lines provides a large number of parameters which directly determine the measuring accuracy. These include:
the exact spatial position between the camera and the projector (triangulation angle, base line, mutual orientation, etc.);
the exact internal parameters of the camera and the projector (focal lengths, sensor dimension, geometry of the picture elements, tilt angle and angle of rotation of the laser line projector, etc.);
the exact spatial positions of the triangulation arrangement for each measuring image taken, the so-called external parameters.
Therefore, calibration of a 3D scanner is a complicated process which is to be carried out with the aid of high-precision calibration bodies and which in many cases is asking too much of the sales staff of, e.g., an orthopedic specialist store and is therefore not accepted.
Because of the mechanical stability required, these 3D scanners cannot be offered at a particularly low price, either, so that many potential applications of the so-called mass-customization (the production of individually fitting clothing and the like) are not currently implemented due to the high costs of the 3D scanners.
The company of corpus.e AG (www.corpus-e.com) has developed a photogrammetric foot scanner under the name “Lightbeam®”, which operates without a projector and thus also without a sensitive triangulation arrangement. Here, the foot is covered with a specially marked, elastic sock and a video camera is mechanically moved around the foot (see also WO 2004/078040 A1). The foot is placed on a photogrammetrically marked support, so that the spatial position from which the camera measures can be permanently automatically determined using the methods of photogrammetry (the so-called “external” parameters of a photogrammetric measuring arrangement). Likewise, the so-called “internal” parameters of the camera itself, such as focal length, image sensor, piercing point of the optical axis, lens distortions, etc. can be determined automatically from the evaluation of overlapping 2D exposures of the marked support and the marked foot. This makes this system completely calibration-free. It may be put into operation after transportation at any time without calibration; there is no need to ever recalibrate it after a change of load; the structure may be of a simple and inexpensive design in terms of mechanical stability since the latter does not contribute to the final result, the 3D model measured.
There is, however, a drawback inherent in this otherwise powerful method: due to the density, which is limited by constitution, of the photogrammetric markings on the elastic sock, the density of the XYZ point cloud generated is distinctly lower in comparison with a laser or stripe projection method (typically 4000 XYZ points as against approx. 1 million XYZ points). While this lower point density does not constitute a disadvantage in the case of flat body parts such as the upper foot, it is critical in regions of high spatial curvatures such as in the region of the toes, the heel, the transition from the upper foot to the sole, etc.
The requirement that the body to be measured needs to be covered with a photogrammetrically marked, elastic covering constitutes a further drawback. Such coverings are not simple to produce; depending on the body part, such as the torso, legs, feet, etc., several shapes and sizes are needed.
It may also be important that, when a customer's feet are digitized for the selection of suitable ski boots, for example, the customer keeps on his/her own winter sock, for the sock to be taken into consideration in the shape adaptation. But it is not possible to photogrammetrically mark any random sock using simple means.
There is therefore a great economic and technical interest in providing a 3D digitizer which does not require any complicated calibration and which generates a density of spatial points without the requirement of using a photogrammetrically marked, elastic covering. The 3D digitizer should thus be cost-efficient, high-resolution, and calibration-free or self-calibrating.